I love Roller Coaster Tycoon.
It's absolutely crazy how he managed to write a game in a way many wouldn't even attempt even in those days, but it's not just a technical feat, it's a creative masterpiece that's still an absolute blast to play.
It still blows my mind how smoothly it gives the illusion of 3D and physics, yet it can run on almost anything.
OpenRCT brings a lot of quality of life and is often the recommended way to play today, but the original RCT will always deserve a spot on any "Best Games of All Time" list.